Release notes for Data Prepper 2.15.1 (#6831)

Signed-off-by: David Venable <dlv@amazon.com>

Author: dlvenable

release/release-notes/data-prepper.release-notes-2.15.1.md

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+## 2026-05-07 Version 2.15.1
+
+---
+
+### Bug Fixes
+
+* Fix `RST_STREAM` non-retryable errors by updating Armeria to 1.32.6 ([#6271](https://github.com/opensearch-project/data-prepper/issues/6271))
+* Fix `JacksonEvent.merge()` skipping keys with explicit null values ([#6819](https://github.com/opensearch-project/data-prepper/pull/6819))